1. Field of the Invention
This invention generally relates to magnetic recording media and more particularly to a magnetic disk having a thin film magnetic layer thereon resistant to thermal erasures of stored information.
2. Description of the Prior Art and Related Information
A huge market exists for hard disk drives for mass-market host computer systems such as servers, desktop computers, and laptop computers. To be competitive in this market, a hard disk drive must be relatively inexpensive, and must accordingly embody a design that is adapted for low-cost mass production. In addition, it must provide substantial capacity, rapid access to data, and reliable performance. Numerous manufacturers compete in this huge market and collectively conduct substantial research and development, at great annual cost, to design and develop innovative hard disk drives to meet increasingly demanding customer requirements.
Each of numerous contemporary mass-market hard disk drive models provides relatively large capacity, often in excess of 2 gigabytes per drive. Nevertheless, there exists substantial competitive pressure to develop mass-market hard disk drives that have even higher capacities and that provide rapid access. Another requirement to be competitive in this market is that the hard disk drive must conform to a selected standard exterior size and shape often referred to as a "form factor." Generally, capacity is desirably increased without increasing the form factor or the form factor is reduced without decreasing capacity.
Satisfying these competing constraints of low-cost, small size, high capacity, and rapid access requires innovation in each of numerous components and methods of assembly including methods of assembly of various components into certain subassemblies. Typically, the main assemblies of a hard disk drive are a head disk assembly and a printed circuit board assembly. The head disk assembly includes an enclosure including a base and a cover, at least one disk having at least one recording surface, a spindle motor for causing each disk to rotate, and an actuator arrangement. The disk typically includes a substrate, an underlayer, a magnetic layer for storing information, and an overcoat layer for protecting the disk. The printed circuit board assembly includes circuitry for processing signals and controlling operations.
Actuator arrangements can be characterized as either linear or rotary; substantially every contemporary cost-competitive small form factor drive employs a rotary actuator arrangement. The rotary actuator arrangement typically includes a head stack assembly, a pivot bearing cartridge, and a permanent magnet arrangement. The head stack assembly typically includes a body portion having a bore for receiving the pivot bearing cartridge, a coil portion extending from the body portion, at least one actuator arm extending from the body portion in an opposite direction from the coil portion, and a head gimbal assembly attached to the actuator arm. The head gimbal assembly typically includes a load beam attached to the actuator arm, a gimbal attached to the load beam and a head attached to the gimbal.
In operation, when reading and writing data from and to the disk, the disk is rotated rapidly by the spindle motor which causes the head to "fly" (hence, the term "flying head") over the recording surface of the disk. Once the head is positioned over a particular track, the head is then used to read or write data on that track. A problem arises from the fact that due to the close proximity of the flying head to the recording surface while the disk is rotating, head disk interferences may occur. Head disk interference includes direct contact between the flying head and disk such as when shock is applied to a disk drive. Head disk interference also includes the instance when a particle is trapped between the flying head and disk such the particle contacts the disk. Such head disk interferences produce frictional heating which momentarily raises the localized temperature of the disk. Such a momentary rise in localized temperature on the disk is referred to herein as a "flash temperature."
In disks having substrates which are good conductors of heat such as an aluminum substrate, flash temperatures do not pose a problem since such flash temperatures are relatively low. However, in a disk having an insulating substrate such as a glass substrate, flash temperatures, if high enough, may erase stored information in the magnetic layer since glass is a poor conductor of heat.